Electro-hydraulic systems or fluid circuits utilize various electrically-actuated and hydraulically-actuated devices, alone or in combination, to provide open-loop or closed loop feedback control. In a closed-loop system in particular, feedback mechanisms or sensors can be used to monitor circuit output values. Each sensor can generate a signal that is proportional to the measured output, and using a suitable control logic device or controller the output can be compared to a particular input or command signal to determine if any adjustments or control steps are required. Sensors for use in an electro-hydraulic fluid circuit ordinarily include pressure transducers, temperature sensors, position sensors, and the like.
In a conventional fluid circuit, the precise control of the operation of the fluid circuit can be maintained by continuously processing the various measured or sensed output values. Supply and tank pressures, as well as pressures operating on particular ports or chambers of a control valve, cylinder, or fluid motor used within the circuit, can be continuously fed to a control unit or controller. However, system control can be lost or severely degraded in a conventional fluid circuit if any of the required pressure or position sensors fails or ceases to function properly for whatever reason. While certain code-based methods exist for detecting out-of-range sensor operation, or for determining shorted or open circuits, such methods usually result in a temporary shutdown of the process utilizing the fluid circuit, and therefore can be less than optimal when continuous fluid circuit operation is required.